The horizontal sync signal applied to the CRT display apparatus varies widely depending on an image to be displayed on a display screen, and it does not have a standardized phase relation with the video signal. As a result, the image position on the display screen varies depending on the frequency of an applied horizontal sync signal, and hence in order to display an image at the center of the display screen for any video signals of different phase relations, the CRT display apparatuses are equipped with a phase adjustment circuit for adjusting the image position on the display screen.
Conventionally, a phase adjustment circuit as shown in FIG. 4, for example, has been used. This conventional phase adjustment circuit comprises a first monostable multivibrator (hereinafter abbreviated as MM) 101 which is triggered by a rising edge of an input sync signal B and produces a pulse signal having a pulse width of .tau..sub.101 so that the input sync signal B is delayed by a desired delay time, and a second monostable multivibrator (hereinafter abbreviated as MM) 102 which is triggered by a falling edge of an output pulse signal of the MM 101 and produces a pseudo sync signal having the same pulse width as that of the input sync signal B. Though it has not been illustrated, both the MM 101 and 102 have a known circuit arrangement for setting the output pulse width by using a time constant determined by a capacitor C and a resistor R, and specifically the MM 101 uses a variable resistor so that its time constant may be varied.
The operation of the conventional phase adjustment circuit using the MM 101 and 102 will be explained with reference to FIG. 5. Shown at (A) in FIG. 5 is a video signal applied to a CRT monitor display, and shown at (B) therein is a sync signal. When triggered by an edge (a rising edge in the case shown at (B) in FIG. 5) of the sync signal B, the MM 101 produces an output as shown at (C) in FIG. 5. The falling edge of the output (C) is used to trigger the MM 102, which then produces a pulse output (D) delayed by .DELTA.t equal to the output pulse width .tau..sub.101 of the MM 101, and the pulse D is used as a pseudo sync signal for effecting the adjustment of the image position on the display screen. However, the MM 101 and 102 are both basically delay circuits, so that they provide an adjustable range only on the delay side, and therefore there is a disadvantage that the image position can merely be moved in one direction on the screen.
In order to overcome this disadvantage, there is a method of producing a pseudo sync signal having an equivalent leading phase by making the pulse width .tau..sub.101 of the output pulse of the MM 101 nearly as wide as the period T by utilizing repetitive generation of the sync signal at the constant period T.
However, there is a problem such that, in this method, an output waveform of a charge/discharge circuit of the MM 101 formed by a capacitor and a resistor has a gentle slope, so that the level of the output waveform varies near a threshold level for triggering a switching element for a relatively long period, which gives rise to a false operation of readily exceeding the threshold level upon mixing-in of noises, resulting in a failure in obtaining an output signal having a desired pulse width.
Then, there is conceived a method of making an output waveform of the charge/discharge circuit have a relatively steep slope by additionally disposing a new monostable multivibrator, which is triggered by a rising edge of the sync signal, for example, and outputs a pulse having a pulse width equal to one half period of the sync signal, at the preceding stage of the MM 101 and 102, thereby reducing a delay caused by the MM 101.
However, as is well known, a monostable multivibrator is formed by a combination of switching elements, e.g. transistors, capacitors and resistors, for example, and its output pulse width cannot be constant due to dispersion of characteristics of these component parts. In addition, the input/output characteristics of the switching elements vary depending on the frequency, amplitude, etc. of the applied sync signal and also vary depending on the temperature. As a result, it is not possible to assure a stable half-period pulse width for an input sync signal of any arbitrary frequency irrespective of temperature changes, etc.
Accordingly, the present invention is intended to overcome the foregoing prior art problems, and it is an object of the present invention to provide a phase adjustment circuit capable of making stable adjustment of the image position on the display screen still with a simple structure.